The scandal involving a former member of the special investigation unit of the presidential office is snowballing. Independent investigation, either by the National Assembly or a special prosecutor, seems inevitable.

The root of the scandal dates to 2009 when Ambassador to Russia Woo Yoon-keun, then a lawmaker of the current ruling party, allegedly received 10 million won ($8,800) from a businessman who sought Woo’s help for getting his relative a job.

Kim Tae-woo, a prosecution investigator who was a member of the presidential special investigation team until recently, filed a report on the case in September last year when Woo, then the secretary-general of the National Assembly, was nominated as ambassador to Russia.

Kim, now sent back to the prosecution and under investigation for alleged misconduct while he was working at Cheong Wa Dae, claims his superiors at the presidential office dismissed the report and disciplined him for filing it. Kim also claimed his report on wrongdoing by Lee Kang-rae, a former lawmaker and head of the Korea Expressway Corporation, was dismissed in a similar way.

In other words, Kim is insisting that presidential officials tried to protect people like Woo and Lee and he was punished for raising issue with ethical problems of close associates of President Moon Jae-in.

Presidential officials said the former investigator was making false allegations out of personal grudge over the disciplinary action taken against him for interfering with a police investigation in which his acquaintance was involved. Prosecution is also looking into allegations that he had corrupt relations with business executives.

Commenting on Woo’s case, Cheong Wa Dae spokesman Kim Eui-kyeom said then the National Assembly secretary-general was not subject to monitoring by the presidential special investigation team.

It is true that the special investigation unit’s job is surveilling senior administration officials, including Cheong Wa Dae staff, and relatives and family members of the president.

But it is wrong to argue that Kim went beyond his duty, because the presidential office is responsible for conducting background investigations of candidates for senior government positions, including ambassadors. It should have been Kim’s job to report a suspected bribery case involving a man who was to take a key ambassadorial post.

In yet another blunder, presidential officials first said the state prosecution investigated the Woo case and cleared him of any wrongdoing. As it turned out, the prosecution did not look into the case. This indicates that Cheong Wa Dae either lied or did not try to ascertain what really happened. There is every reason to investigate the case thoroughly.

If the Woo case has again raised questions about the faulty vetting process for senior administration officials, Kim came up with a potentially more explosive allegation: The Moon administration was illegally surveilling civilians.

In one case, Kim claimed, he gathered information about whether the son of Goh Kun, who served as prime minister during the Roh Moo-hyun administration, and other former senior administration officials possessed cryptocurrency. He also said his team tracked executives of the airport railroad corporation and some heads of commercial banks. All these, if they prove true, violate the law.

Presidential officials deny the allegations too. They say Cheong Wa Dae collected information about cryptocurrencies to formulate a policy to cope with a possible bust of the digital coin boom. That does not sound convincing.

Regarding the surveillance on airport railroad corporation officials, Cheong Wa Dae said it did not know the corporation was not a public enterprise and thus was not subject to government scrutiny. This is even more unconvincing.

The unalterable fact is that -- intentionally or mistaken -- some members of the presidential special investigation unit gathered information illegally. There is a strong possibility that Kim’s former colleagues did the same.

We are all familiar with cases like the Woo scandal and the allegations about government surveillance of civilians. The best solution we have learned from the past cases is to have independent investigation, by the National Assembly or a special prosecutor.